1. Field of the Invention
The present invention relates to an obstacle detection apparatus which prevents the collision of a vehicle.
2. Description of the Related Art
Conventionally, there has been proposed an obstacle detection apparatus which detects presence of on obstacle behind a vehicle and informs a person in the vehicle with an alarm. The obstacle detection apparatus has an ultrasonic sensor on a rear bumper of a vehicle, and receives an echo of the ultrasonic wave emitted from that sensor, hit on and reflected by the obstacle, thereby detecting the obstacle behind the vehicle from that echo.
In a four-wheel drive vehicle, however, when an openable/closable body like a tailgate is opened, or when a tire, a ski carrier, etc. are placed at the rear of the vehicle, the distance from the ultrasonic sensor to the obstacle differs from the distance from the rearmost part of the vehicle to the obstacle. This raises a problem that the vehicle has already come closer to the obstacle than is realized by the person in the vehicle, at the time the obstacle is detected by the ultrasonic sensor.
To resolve such a problem, Japanese Unexamined Patent Publication No. 2002-318282 (Patent Literature 1) discloses an obstacle detection apparatus which changes the maximum detection distance of a sensor section, or, specifically, increases the maximum detection distance in accordance with a distance between an ultrasonic sensor and the rearmost part of a vehicle, thereby informing presence of an obstacle early to allow a driver to maneuver the vehicle to avoid the obstacle. Japanese Unexamined Patent Publication No. 6-242238 (Patent Literature 2) discloses a vehicular obstacle detection apparatus which corrects the distance from a sensor section to an obstacle to a proper value for avoiding the obstacle.
According to the technology of Patent Literature 1, a person in a vehicle (driver) backs the vehicle while usually checking the distance to the obstacle. When that distance becomes a shorter, or, when an alarm starts beeping because of the detection of presence of the obstacle in the maximum detection distance, the driver carefully backs the vehicle with the help of that beeping alarm, thereby avoiding a collision with the obstacle. However, in a case where there is a difference between the distance from the ultrasonic sensor to the obstacle and the distance from the rearmost part of the vehicle to the obstacle, when the maximum detection distance changes, the alarm starts beeping at a point where the obstacle is yet sufficiently away from the rearmost part of the vehicle. Accordingly, the driver may feel awkward, and may have to pay unnecessary attention. According to the technology of Patent Literature 2, the distance from the detector to the obstacle is corrected for avoiding the collision of a bumper with the obstacle. Because the alarm sound is constant, however, the driver cannot know how close to the obstacle the vehicle has come from that alarm sound.
The invention has been made in view of the above-described problems. It is an object of the invention to provide an obstacle detection apparatus which can accurately inform a person in a vehicle of the distance to an obstacle without giving awkward feeling to the person in the vehicle.